1. Field of the Invention
The invention relates generally to threaded connections used for couple segments of pipe. More specifically, the invention relates to structures for threaded connections that may be used particularly in radially, plastically expandable tubes or pipes.
2. Background Art
Threaded tubular connections are used for joining segments of conduits or pipes end-to-end to form a continuous conduit or pipe for transporting fluid under pressure. Oilfield tubular goods, such as casing, generally use such threaded connections for connecting adjacent sections of conduit or pipe. Examples of such threaded connections designed for use on oilfield tubular goods are disclosed in U.S. Pat. Nos. 2,239,942; 2,992,019; 3,359,013; RE 30,647; and RE 34,467.
In U.S. Pat. No. RE 30,647 issued to Blose, a particular thread form or structure is disclosed for a tubular connection that provides an unusually strong joint while controlling the stress and strain in connected “pin” (male thread) and “box” (female thread) members to within acceptable levels. The pin member has at least one generally dovetail-shaped external thread whose width increases in one direction along the pin, while the box member has at least one matching generally dovetail-shaped internal thread whose width increases in the other direction. The mating set of dovetail-shaped threads provide a wedge-like engagement of opposing pin and box thread flanks that limit the extent of relative rotation between the pin and box members, and define a forcible make-up condition that completes the connection. In this thread structure, the angles of the flank shoulder, as well as the thread width, can be used to control the stress and strain preload conditions induced in the pin and box members for a given make-up torque. Thus, by tailoring the thread structure to a particular application or use, the tubular connection or joint is limited only by the properties of the materials selected.
A prior art tubular threaded connection includes a pin member and a box member. The box member has a tapered, internal, in many cases generally dovetail-shaped thread structure formed thereon which is adapted for engaging complementary tapered, external, thread structure formed on the pin member to mechanically secure the box and pin members in a releasable manner.
Internal thread on the box member has stab flanks, load flanks, roots, and crests. The internal thread increases in width progressively at a uniform rate in one direction over substantially the entire helical length of thread. External thread on the pin member has stab flanks, load flanks, roots, and crests. The external thread increases in width progressively at a uniform rate in the other direction, over substantially the entire length of the external thread. The oppositely increasing thread widths, and the taper of threads cause the complementary roots and crests of the respective threads and to move into engagement during make-up of the connection in conjunction with the moving of complementary stab and load flanks into engagement upon make-up of the connection.
The pin member or the box member defines the longitudinal axis of the made-up connection. The roots and crests of the box and pin members in some cases are flat and parallel to the longitudinal axis of the connection and have sufficient width to prevent any permanent deformation of the threads when the connection is made up.
An important part of any connection is a seal for keeping the conduit fluid pressure-tight at the connections. Typically connections will be designed to include metal-to-metal seals therein. Metal-to-metal seals have the advantage of not requiring gaskets or other additional sealing devices, which would typically have to be replaced periodically as the connections are coupled and uncoupled. Metal seals are created when contact pressure between two metal surfaces exceeds the fluid pressure to be sealed. Typically the contact pressures are created during make up of the connection. Some types of metal to metal seal are energized by internally pressurizing the conduit.
More recently, oilfield tubular goods have been developed which can be plastically radially expanded from their initial diameters after being installed for the intended application. See for example, R. D. Mack et al, How in situ expansion affects casing and tubing properties, World Oil, July 1999, Gulf Publishing Co., Houston, Tex., for a description of radially expandable oilfield tubular goods. Radially expandable tubular goods have particular application as casing in oil and gas producing wells. It has been difficult to seal radially expandable tubular connections using metal-to-metal seals known in the art.
It has also been determined that conventional threaded connections, including the previously described variable width threads, undergo large changes in distribution of stresses when such couplings are radially, plastically expanded. It is desirable to have a threaded connection which can maintain strength and sealing ability even after plastic radial expansion. It has also been determined that threaded connections such as the previously described variable thread width connections may have uneven stress distribution when the connection undergoes substantial compressive or tensile stress. It is therefore desirable to have a threaded connection which is better able to resist tensile and compressive stresses.